A moonphase watch tracks the current phase of the moon as it appears in the sky. The entire cycle of the moon takes 29 ½ days, roughly the average span of our calendar month. Accounting for the discrepancy of the measured lunar cycle vs. actual moon phases, watchmakers created an advanced mechanism that ensures the complication will only be off by 1 day every 122 years.

Once a very useful feature for sailors to gauge the tides, the moonphase is now less practical and more of a frivolous, but very cool, attribute for a luxury watch. High-end watch brands like Baume & Mercier, Girard Perrageaux, Jeager-LeCoultre, and Patek Phillippe incorporate moonphase into many of their models.